Combine the flour, cocoa powder, baking powder, baking soda, nutmeg, espresso powder and salt. Set aside.
Combine melted butter, buttermilk*, vanilla extract, egg yolk, white sugar and brown sugar. Mix well.
Add the wet ingredients to the dry ingredients. Mix well.
Preheat oven to 350 degrees. Spray a donut pan with cooking spray.
Baked mini donuts for 6-8 minutes and regular sized donuts for 8 minutes.
Remove from oven and let cool in pan for 2 minutes. Then turn pan upside down and put donuts on a cooling rack.
Repeat process with second batch if using just one pan.